1. Field of the Invention
The present invention relates to the field of polymers, and more particularly, to polymer dielectrics for use in electronic applications.
2. Background Information
There are a wide variety of polymer materials which may be used as dielectric layers and for other purposes in electronic and other applications. Each polymer composition has a set of physical properties such as density, water absorption tendency, dielectric constant, melting point or softening point, color, susceptibility to laser ablation, and so forth. A high density interconnect (HDI) system is disclosed in U.S. Pat. No. 4,783,695 to C. W. Eichelberger et al. Methods of fabricating such high density interconnect structures are disclosed in U.S. Pat. Nos. 4,714,516 and 4,835,704 to C. W. Eichelberger et al. This high density interconnect structure comprises integrated circuit chips having contact pads thereon mounted on a substrate with a dielectric layer adhesive bonded thereover. Via holes are formed through the dielectric layers and a patterned metallization layer is disposed on top of the dielectric layer and extends into the via holes to make electrical contact to the contact pads of the circuit chips.
This structure places special requirements on the dielectric materials. In particular, in order for the final structure to be usable over a wide temperature range, the dielectric layers must have high melting points and high thermal stability. They must also be laser ablatable by ultraviolet light in order to form the via holes through which different layers of metallization are connected. In the HDI system, laser processing (ablation, photoresist exposure, etc.) is normally done with one, or at most, two passes of the laser beam with a power ranging from 0.5 to 2.0 watts with a preferred maximum power level being about 1.5 watts. Thus, when a dielectric layer is characterized as being laser ablatable, it means that such a layer can be totally removed by one or two passes of a laser beam of this power level and when it is characterized as not being laser ablatable, it means that a layer is not completely removed by one or two passes of such a laser beam.
To minimize the complexity and cost of equipment for fabricating such high density interconnect structures, it is considered desired to be able to do all laser processing at a single frequency in order that only a single laser is required. Accordingly, preferred materials are those which may be processed at a laser frequency of 351 nm. This frequency was selected in accordance with the characteristics of desirable dielectric layers such as KAPTON.RTM. polyimide available from DuPont de Nemours and the fact that there are commercial photoresists which can be processed at this frequency. ULTEM.RTM. polyetherimide resin available from General Electric Company has been used as an adhesive layer in this high density interconnect structure for bonding KAPTON.RTM. to the underlying structures. The ULTEM.RTM. resin is laser ablatable at 351 nm. The ULTEM.RTM. material has a melting point in the neighborhood of 220.degree. C. or higher, depending on its specific formulation. This ULTEM.RTM. high temperature adhesive layer is suitable for use in permanent structures.
The high density interconnect structure referred to above has many fine line electrical conductors in its metallization pattern. These electrical conductors preferably range from 1/2 mil to 2 mil in width and may be as long as several inches. The dielectric layers in this HDI structure are typically 1/2 mil to 1 mil thick. During the fabrication process, errors in the metallization pattern can be corrected without additional rework if they are discovered and corrected before the next dielectric layer is applied on top of the metallization layer containing the errors. Metallization which is disposed in a location where no metallization is desired may be removed by direct laser ablation, photoassisted etching operations or by the use of photoresist and plasma or wet etching of the undesired metal. Where gaps in the desired metal are present, additional metal may be deposited in those locations by laser induced deposition by decomposition of organometallic compounds where the deposition takes place only where the laser illuminates the dielectric layer.
If errors in the metallization pattern are not discovered until after deposition of successive dielectric and possibly metallization layers, then the structure may be repaired by removing the entire dielectric overlay structure and starting over. It is this repair process which must be used when errors in the metallization pattern are not found until electrical testing of the overall structure is undertaken. Consequently, there is the desire to accurately inspect each metallization layer prior to the application of the next dielectric layer. Printed circuit boards which have conductors which are typically 5 to 10 mils wide and may be as long as 20 inches or so, and have dielectric layers typically 2 to 7 mils thick, that is, 2 to 14 times thicker than the dielectric layers in an HDI structure can be inspected by using the model 2020 inspection apparatus which is commercially available from Lincoln Laser in Phoenix, Ariz. Unfortunately, such testing apparatus is at present incapable of properly inspecting HDI layers because of their extremely narrow conductors. The above Model 2020 inspection system relies on inherent luminescence of the dielectric materials used in circuit boards to produce a fluorescent output which is sufficient to enable the system to clearly distinguish between conductors and dielectric material. Many of the dielectric materials which are suitable for use in an HDI system are either non-fluorescent or so weakly fluorescent as to not provide a usable contrast ratio between dielectric and metal regions in the thicknesses in which those dielectrics are used in an HDI structure.
Unfortunately, because of the low level of fluorescence present in some of the dielectric layers in an HDI structure, this equipment is incapable of inspecting such structures accurately.
Consequently, improved methods of inspection for fine line multilayer conductor structures of the type present in an HDI system are needed.